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Recently, a silicon nitride based cutting tool, having superior mechanical strength and resistance against thermal shock, has come into use as a tool for cutting cast iron. On the other hand, under more stringent cutting conditions, such as high-speed wet machining with a cutting speed exceeding 800 m/min, sufficient resistance against thermal impact or wear resistance cannot be sufficiently assured even with the use of the silicon nitride based cutting tool. In such case, the CBN cutting tool is used. However, the CBN cutting tool, manufactured in general using an ultra high pressure sintering method, is much more costly than the silicon nitride based cutting tool. Thus, various proposals have been made for a silicon nitride based cutting tool for obtaining a tool more inexpensive and more suitable for high-speed machining of cast iron.
In the JP Patent Kokai JP-A-58-95662 and JP Patent Kokoku JP-B-5-7122, there is disclosed a tool in which TiN is compounded as a hard dispersed phase in silicon nitride (Si.sub.3 N.sub.4) to suppress the chemical reaction between silicon nitride and Fe at elevated temperatures. In JP Patent Kokoku JP-B-61-10416 and JP Patent Kokai JP-A-61-17473, there is proposed a tool in which titanium nitride, titanium carbide or titanium carbo-nitride is compounded as a hard dispersed phase into silicon nitride for improving wear resistance. On the other hand, in JP Patent Kokai JP-A-61-31558, there is proposed a tool in which, for suppressing generation of silicon carbide or titanium nitride due to chemical reaction of titanium carbide with silicon nitride and for suppressing generation of N.sub.2 gas, the titanium carbide particle surface is coated with titanium nitride and compounded as a hard dispersed phase with silicon nitride.